Diffuse reflectance spectroscopy is a well known technique especially useful for analyzing samples of materials that are difficult to analyze by classical techniques. Powders and rough surface solids are examples of samples that can be well handled by diffuse reflectance spectroscopy. Various optical geometries for this purpose have been described in the literature. See, for instance, the article by Brimmer, Griffiths and Harrick on pages 258-265 of Vol. 40, No. 2 , 1986 of Applied Spectroscopy, and the references cited therein. The aforementioned article is herein incorporated by reference. As described in that article, a number of diffuse reflectance accessories have been developed commercially to fit into the sample compartments of most of the Fourier Transform Infrared (FT-IR) spectrometers as well as the older dispersive-type spectrometers. A typical accessory comprises a base or support plate which can be seated in the sampling compartment of the spectrometer. The base supports one or more plane mirrors for receiving the incoming beam from the spectrometer and redirecting it to a first focussing mirror which focusses the beam at a selected angle onto the sample surface. Typically, the sample is a powder pressed into a recess on a post also mounted on the base. The diffuse reflections from the sample are collected by a second focussing mirror which passes the reflected beam to one or more plane mirrors which in turn redirect the beam back into the spectrometer so it can impinge on the usual detector which converts the beam into an electrical signal. The electrical signal is processed in the standard fashion to produce, typically, a graph which relates the signal intensity to the wavelength of the incident beam. The diffuse reflection is a function , among other things, of the sample absorptivity and thus the output is typically plotted as %-transmittance as a function of wavelength or more commonly wavenumbers. One popular diffuse reflection accessory is manufactured by Harrick Scientific Corp. and is widely known as the "Praying Mantis" Model due to the configuration of the focussing mirrors which are large ellipsoid mirrors positioned over the sampling post.
Diffusely reflected radiation is spread out in the half of the full solid angle measured from the sample surface. Though the second ellipsoidal mirror is a large segment, a considerable amount of diffusely reflected radiation from the sample escapes uncollected. This reduces the quality of the output. In addition, for poorly reflecting samples, the signal-to-noise (S/N) ratio is poor.